발간논문

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Hot-pressing of cast Pr-Fe-B-Cu alloys was carried out as a new manufacturing process for R (Pr)-Fe-B permanent magnets. And the effects of hot-pressing variables and heat treatment on the magnetic properties of Pr-Fe-B-Cu magnets were investigated. Pr-Fe-B-Cu ingot consists of Pr₂Fe_(14)B, Pr rich phase and elemental Fe. It is thought that Fe diffused into Pr₂Fe_(14)B phase during heat treatment at 1000℃ or hot-pressing of Pr-Fe-B-Cu alloys. Hot-pressing of Pr-Fe-B-Cu alloys induced magnetic anisotropy to the direction of hot working. The most optimal strain rate for hot-pressing of Pr-Fe-B alloys was ε˙=0.001/sec.

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Single crystal growing of YBCO superconductor was attempted using a BaO-CuO flux method. 123 superconducting powders mixed with 28 BaCO₃-72 CuO flux in variable compositions were held at 1030℃ for 10 hours and cooled at the rate of 5∼30℃/hr. In the proper composition of melt and cooling rate, YBCO single crystals with the mean size of 1.5 × 1. 5 × 0.1㎣ could be obtained and had grown on the wall of crucible up to the size of 2.5 × 3 × 0.1㎣. YBCO single crystals had a shape of very thin plate with well developed (001) crystal face and were characterized by EDS, XRD, and Laue method.

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Effect of hold time on the low cycle fatigue life of 12%Cr rotor steel was studied at 873K in air atmosphere. In order to investigate the effect of creep-fatigue interaction, hold time of 10 min was applied at tensile, compressive and tensile-compressive peak strain, respectively. The experimental results show that the life of creep-fatigue cycling is shorter than that of continuous cycling. For all the tests except tensile hold time cycling, the transgranular mode fracture was observed and no grain boundary creep cavites were shown. In case of tensile hold time cycling, mixed mode(inter and transgranular) fracture was observed and creep cavities were shown at grain boundaries and lath martensite boundaries. However, regardless of hold mode, the hold time cycling decreased the fatigue endurance. From the experimental results, the decrease in the fatigue life with hold time can be explained as follows : Oxidation during hold time decreases the number of cycles for crack initiation. Since the recovery during hold time reduces the flow stress, the crack tip opening displacement is enlarged. By that way, the crack growth rate is increased. Regardless of hold mode, two effects mentioned above reduced the fatigue life. Especially, in the case of tensile hold time cycling, due to the fatigue crack interactions with creep cavities, the fatigue life is significantly decreased.

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Adhesion and leakage current characteristics of selective CVD W by SiH₄ reduction on the Si substrate have been investigated. Adhesion of the W film on to the Si substrate (damaged by ion implantation) turned out to be very poor. Adhesion property also appears to depend on W film thickness. In the case of the W film with the thickness of 1500Å, adhesion is poor even after removing all the implantation damage from the Si substrate. Two-step deposition process of W film for enhancing adhesion have been investigated. For the two-step deposition process where H₂ reduction is used in the first step and SiH₄ reduction is used in the second step, leakage current due to encroachment phenomena is very high. On the other hand, for the two-step process where SiH₄ reduction is used in the first step and then SiH₄ reduction is used again in the second step but in situ annealing treatment was inserted in between steps, both adhesion and electrical properties of the W film is found to be good, even if the W film thickness was over 5000Å.

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A new method of diffusion bonding(named as Melting Induced Diffusion Bonding) which aimed to make better Transient Liquid Phase(TLP) bonding was developed and applied to the bonding of 304/304 stainless steel. On a new method, boron layer was used as a insert material and its melting point is higher than that of the base metal, therefore insert doesn`t melt during bonding, which is different from TLP bonding and brazing. Base metal is bonded by melting of base metal itself which is caused by the reaction of base metal with boron during bonding. By applying a new bonding method to 304 stainless steel, we obtained 421㎫ tensile strength and 17% elongation of the bonded joint.

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